1. Field of the Invention
The invention is directed generally to a modification of the standard hydraulic circuitry which uses double-acting cylinders in the lift mechanism of mobile material handling equipment, such as front- or rear-end loading waste disposal vehicles. In particular, the invention relates to hydraulic circuitry in such material handling equipment including an electrically operated dump valve in the fluid return line of the double-acting cylinders to cause to them to act temporarily as single-acting cylinders during the measurement of weight-proportional signals from transducers operatively mounted to the cylinders to measure pressure in the lift line thereof. By providing fluid in the cylinder return line with a low resistance pathway, one avoids the pressure variations which are characteristic of double-acting cylinders and detrimental to obtaining accurate measurements of the load applied to the lifting forks of the equipment.
2. Prior Art
In certain mobile equipment for lifting heavy loads, for example fork lifts, a lifting assembly using "single-acting" hydraulic lift cylinders may be employed. In such equipment, the cylinders are called upon either to pull or push the lifting arms, but not both, gravity return being relied on to lower the load.
In a single-acting cylinder, hydraulic fluid force is exerted against one side only of the piston during lifting, forcing the piston rod to extend from the cylinder housing. The cylinder is dry on the opposite side of the piston. When the hydraulic fluid pressure between the lift control valve and the lift cylinder, i.e. the back-pressure, is released the weight of the load and lifting arms causes the piston rod to retract.
It is well known to provide pressure transducer means (e.g. a load cell) at the inlet of the hydraulic lift cylinders (i.e. in the lift circuit) in mobile lifting equipment to produce an electrical signal which measures the back-pressure developed when the lifting arms bearing the load are at some suitable intermediate position, the back-pressure in turn being simply related to the weight of the load. The use of analog/digital converter means for converting transducer-generated analog signals corresponding to the fluid pressure on the transducer means in mobile lifting equipment is illustrated, for example, in the refuse collection vehicle of U.S. Pat. No. 5,038,876 (Smith).
In many front-end loaders, the hydraulic system utilizes double-acting cylinders for better lifting and lowering control and more rapid functioning in the return cycle. The pivotal lifting arms of refuse collection vehicles, as a principal example, must lift the load of waste up and over through more than 90.degree. to transfer collected solid waste from the container into the hopper of the dumpster, so that double-acting cylinders must be used. Hydraulic pressure fluid exerts force on one side of the piston when raising and dumping the waste container and on the other side of the piston to bring the lift arms back and lower the empty container to the ground.
Accuracy in weighing a container picked up by a refuse vehicle when full, and then when empty, is of critical importance in proper waste management. Unfortunately, the configuration of double-acting lift cylinders, having hydraulic fluid on both sides of the its piston, results in the measured back-pressure on the lift circuit side not being reproducible for equal loads held at equal elevations, by reason of variability in the cylinder pressure on the opposite (return line) side of the cylinder. This variability depends, at least, upon the speed at which the equipment operator raises the load to a weighing position using the hydraulic control valve and upon the operating speed of the engine driving the hydraulic pump.
It is an object of the present invention to provide a modification of the hydraulic control system of a front-end loader or similar loading equipment employing double-acting cylinders in the lift mechanism, which provides for enhanced accuracy in weighing a load carrier or container by eliminating the aforementioned variability in cylinder pressure as a source of inconsistent back-pressure readings.
It is a further object of the invention to provide for automated weighing of the load picked up by a loader in conjunction with an on-board microprocessor having peripheral features for controlling of the hydraulic system during the weighing operation and for displaying a load weight reading through calibrated signal conversion means coupled to the output of a transducer responsive to the hydraulic pressure in the lift circuit of the hydraulic lift cylinder of the loader.